Examining the Effect of Pore Size Distribution and Shape on Flow through Unsaturated Peat using Computer Tomography

The hydraulic conductivity of unsaturated peat soil is controlled by the air-filled porosity, pore size and geometric distribution as well as other physical properties of peat materials. This study investigates how the size and shape of pores affects the flow of water through peat soils. In this study we used X-ray Computed Tomography (CT), at 45μm resolution under 5 specific soil-water pressure head levels to provide 3-D, high-resolution images that were used to detect the inner pore structure of peat samples under a changing water regime. Pore structure and configuration were found to be irregular, which affected the rate of water transmission through peat soils. The 3-D analysis suggested that pore distribution is dominated by a single large pore-space. At low pressure head, this single large air-filled pore imparted a more effective flowpath compared to smaller pores. Smaller pores were disconnected and the flowpath was more tortuous than in the single large air-filled pore, and their contribution to flow was negligible when the single large pore was active. We quantify the pore structure of peat soil that affects the hydraulic conductivity in the unsaturated condition, and demonstrate the validity of our estimation of peat unsaturated hydraulic conductivity by making a comparison with a standard permeameter-based method. Estimates of unsaturated hydraulic conductivities were made for the purpose of testing the sensitivity of pore shape and geometry parameters on the hydraulic properties of peats and how to evaluate the structure of the peat and its affects on parameterization. We also studied the ability to quantify these factors for different soil moisture contents in order to define how the factors controlling the shape coefficient vary with changes in soil water pressure head. The relation between measured and estimated unsaturated hydraulic conductivity at various heads shows that rapid initial drainage, that changes the air-filled pore properties, creates a sharp decline in hydraulic conductivity. This is because the large pores readily lose water, the peat rapidly becomes less conductive and the flow path among pores, more tortuous

The applicability and accuracy of computer modeling in regards to acoustical scattering by a complex geometry

Thesis: S.B., Massachusetts Institute of Technology, Department of Architecture, 2005.Cataloged from PDF version of thesis.Includes bibliographical references (page 27).The intent of the investigation is to try to characterize the nature of scattered acoustical energy off of the face of a concrete masonry unit with an atypical geometry. The nature of the tests conducted would be in accordance with the AES-4id-2001 document which pertains to the Characterization and measurement of surface scattering uniformity. The uniformity of scattering can be analyzed and can give one an indication of the diffusive properties of the test samples. The product for which the testing is proposed, as previously mentioned, is a modification of a concrete masonry unit. The product is not uniform in section, a fact which means a two dimensional analysis of scattering will not suffice. Instead, the distribution of reflected sound waves over a hemispherical shell will be examined.by William J. Elliot.S.B

Effects of DTM Resolution On Slope Steepness And Soil Loss Prediction On Hillslope Profiles

Topographic attributes play a critical role in predicting erosion in models such as the Water Erosion Prediction Project (WEPP). The effects of four different high resolution hillslope profiles were studied using four different DTM resolutions: 1-m, 3-m, 5-m and 10-m. The WEPP model used a common scenario encountered in the forest environment and the selected hillslope profiles to calculate the average annual runoff, average annual soil loss and average annual sediment delivery. The DTM resolution affects the slope steepness as well as the erosion and sediment delivery predicted by WEPP. The slope steepness values generated from higher resolution DTMs were less than from lower resolution DTMs. The trends in predicted average annual soil loss as a function of DTM resolution showed the same pattern as for slope steepness

Effects of DTM Resolution On Slope Steepness And Soil Loss Prediction On Hillslope Profiles

Topographic attributes play a critical role in predicting erosion in models such as the Water Erosion Prediction Project (WEPP). The effects of four different high resolution hillslope profiles were studied using four different DTM resolutions: 1-m, 3-m, 5-m and 10-m. The WEPP model used a common scenario encountered in the forest environment and the selected hillslope profiles to calculate the average annual runoff, average annual soil loss and average annual sediment delivery. The DTM resolution affects the slope steepness as well as the erosion and sediment delivery predicted by WEPP. The slope steepness values generated from higher resolution DTMs were less than from lower resolution DTMs. The trends in predicted average annual soil loss as a function of DTM resolution showed the same pattern as for slope steepness

Artificial intelligence and learning environments: Preface

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28724/1/0000545.pd

Structure of the Bacterial Cellulose Ribbon and Its Assembly-Guiding Cytoskeleton by Electron Cryotomography

Cellulose is a widespread component of bacterial biofilms, where its properties of exceptional water retention, high tensile strength, and stiffness prevent dehydration and mechanical disruption of the biofilm. Bacteria in the genus Gluconacetobacter secrete crystalline cellulose, with a structure very similar to that found in plant cell walls. How this higher-order structure is produced is poorly understood. We used cryo-electron tomography and focused-ion-beam milling of native bacterial biofilms to image cellulose-synthesizing Gluconacetobacter hansenii and Gluconacetobacter xylinus bacteria in a frozen-hydrated, near-native state. We confirm previous results suggesting that cellulose crystallization occurs serially following its secretion along one side of the cell, leading to a cellulose ribbon that can reach several micrometers in length and combine with ribbons from other cells to form a robust biofilm matrix. We were able to take direct measurements in a near-native state of the cellulose sheets. Our results also reveal a novel cytoskeletal structure, which we have named the cortical belt, adjacent to the inner membrane and underlying the sites where cellulose is seen emerging from the cell. We found that this structure is not present in other cellulose-synthesizing bacterial species, Agrobacterium tumefaciens and Escherichia coli 1094, which do not produce organized cellulose ribbons. We therefore propose that the cortical belt holds the cellulose synthase complexes in a line to form higher-order cellulose structures, such as sheets and ribbons

Robustness of slow contraction to cosmic initial conditions

We present numerical relativity simulations of cosmological scenarios in which the universe is smoothed and flattened by undergoing a phase of slow contraction and test their sensitivity to a wide range of initial conditions. Our numerical scheme enables the variation of all freely specifiable physical quantities that characterize the initial spatial hypersurface, such as the initial shear and spatial curvature contributions as well as the initial field and velocity distributions of the scalar that drives the cosmological evolution. In particular, we include initial conditions that are far outside the perturbative regime of the well-known attractor scaling solution. We complement our numerical results by analytically performing a complete dynamical systems analysis and show that the two approaches yield consistent results.Comment: 41 pages, 18 figures; accepted for publication in JCA